Chromatography is a set of techniques for separating a mixture into its constituents. Generally, in a liquid chromatography analysis, a pump takes in and delivers a composition of liquid solvents at high pressure to a sample manager, where a sample (i.e., material under analysis) awaits injection into the mixture. Disposed between the pump and sample manager, a mixer blends the liquid solvents into a homogenous composition. From the sample manager, the resulting composition comprised of the mixture of liquid solvents and injected sample moves to a point of use, such as a column of particulate matter. By passing the composition through the column, the various components in the sample separate from each other at different rates and thus elute from the column at different times. A detector receives the elution from the column and produces an output from which the identity and quantity of the analytes may be determined
High-performance liquid chromatography (HPLC) uses two basic elution modes: isocratic elution and gradient elution. In the isocratic elution mode, the mobile phase, comprised of either a pure solvent or a mixture of solvents, remains the same throughout the chromatography run. In the gradient elution mode, the composition of the mobile phase changes during the separation. Creation of the gradient involves the mixing of multiple solvents, the proportions of which change over time in accordance with a predetermined timetable. Some HPLC systems create the gradient under high pressure, by mixing the solvents downstream, on the outlet side of the pumps. Such HPLC systems are referred to herein as high-pressure gradient systems. Other HPLC systems create the gradient under low pressure, using a gradient proportioning valve to select from up to four solvents, combining the multiple solvents on the intake side of a single aspirating pump, and changing the proportions of the solvents over time. Such HPLC systems are referred to herein as low-pressure gradient systems.
The choice between a high-pressure and a low-pressure gradient system involves a variety of tradeoffs. For one, high-pressure gradient systems have lesser dwell volumes than low-pressure gradient systems because the solvent mixing occurs after the pumps instead of before the intake side of the pump. On the other hand, low-pressure gradient systems can produce a gradient with just one pump, whereas high-pressure gradient systems generally require one pump for each solvent. Hence, low-pressure-gradient systems are more amenable than high-pressure gradient systems to tertiary and quaternary gradients, and, thus, find use predominantly in such chromatography applications, whereas high-pressure gradient systems generally involve binary gradients.
The output stream of solvent composition produced by low-pressure and high-pressure gradient systems typically has detectable perturbations in a chromatographic baseline, referred to as compositional noise. When a gradient pump outputs a mixture of two fluids, frequencies of operation manifest as oscillations in the compositional output.
A conventional approach for reducing compositional noise is to couple a large-volume mixer to the output of the pump system. This mixer, however, may add an undesirable amount of delay volume to the chromatography system, which can affect the delivery of accurate and reproducible gradients and negatively affect cycle time for a liquid chromatography system. Furthermore, the mixer may actually be ineffective in adequately reducing the compositional noise.